[c-nsp] BGP (VPNv4) flapping every 3minutes

Gert Doering gert at greenie.muc.de
Sun Dec 21 06:48:07 EST 2014


On Sun, Dec 21, 2014 at 08:38:00PM +1100, CiscoNSP List wrote:
> > This hints at a MTU problem in your network, and the proper fix is to
> > fix the MTU problem, not add config to the RR to work around it.
> Thanks Gert - It's because we are purchasing inter-pop links from
> other carriers(And there supported MTU's varies dramatically)...in
> this instance our PE's "agreed" on 1900'odd MSS, but we were only
> able to get ~1544.....On the links we control, it's all 9100.....and
> we tweak as needed with carrier supplied links.

Yeah.  Fun with providers with not clearly defined MTU on such a link
(we have one that changes MTU whenever they move between primary and
backup path...  but the difference is much larger than "just 1 or 2

Even more important to actually ensure that the devices connected to that
link are configured with the correct MTU so everything else can rely
on maximum-sized packets to either go through, or get back an ICMP error
(and adjust).

What we do in our network: we ping (from a FreeBSD host) all devices that
sit behind "interesting" links with 10000 byte packets - knowing that the
packets will be fragmented on the way *to* the device, and same for the
response packets.  If the MTU setting for a link is right, the fragments
will make it.  If the router has a too-high IP MTU, the fragments will
get lost, and the monitoring system will tell us.

Somewhat of a hack, but works.

USENET is *not* the non-clickable part of WWW!
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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